This is not really a bug, more like a difference between browser implementations. The CanIUse site you linked to also gives the solution to make the Three.js examples you referred to work in Firefox: you simply need to set the dom.importMaps.enabled flag in about:config.
So, just open a new tab in Firefox, type about:config in the address bar, look for importMaps and double click it to set it to true. Refresh the Three.js examples link and it will work - it does for me as well, after setting up Firefox like this.
P.S. I don’t know about Safari, never used it, never owned or will own a Mac, and so on. Maybe someone else could cover this part, if a solution exists in this case, that is.